Abstract Pseudomonas aeruginosa?induced corneal keratitis is a sight-threatening disease. The rising antibiotic resistance among the Pseudomonas isolates makes treatment of this disease challenging, justifying the need for alternative therapeutic modalities. We propose to explore the concept that exposure of ocular surfaces to commensal strains or perturbations of commensal ecosystems alters the bactericidal capability of neutrophils against P. aeruginosa, thereby determining the threshold for susceptibility to infection. Specific Aim 1. Define the impact of ocular exposure to commensal strains on susceptibility to P. aeruginosa-induced keratitis. We demonstrated that there was a measurable and significant impact of ocular exposure to commensal organisms that strengthens the immune responses to P. aeruginosa-induced keratitis. In this aim, we will colonize conjunctival mucosal surfaces with different commensal organisms and evaluate the consequences of colonization on P. aeruginosa-induced keratitis. Specific Aim 2. Define the impact of gut microbiota on neutrophil bactericidal activities at baseline and during P. aeruginosa-induced keratitis. Reduction of gut microbiota has a pronounced impact on susceptibility to keratitis, signifying the existence of a mechanism operating along a ?gut-eye? axis. In this aim we will identify gut commensal communities that regulate neutrophil bactericidal properties against P. aeruginosa. We will utilize selective antibiotic-based reduction of commensal organisms and colonization of germ free mice with different commensal communities to evaluate how gut commensals affect neutrophil bactericidal properties and, thereby, cause susceptibility to keratitis. We will leverage our experience in quantitative transcriptomic and proteomic analysis to gain insights into microbiota-driven granulocyte maturation and examine the mechanisms thereof. Specific Aim 3. Evaluate the impact of MIF deficiency on commensal presence and development of keratitis. MIF deficiency confers resistance to P. aeruginosa-induced keratitis and, excitingly, commensal communities in MIF-deficient mice are more diverse than those of MIF-sufficient mice. We will explore how these alterations in commensal presence affect resistance to infection by decreasing commensal abundance either locally or in the gut through selective antibiotic treatments.